The present invention relates to tumor segmentation in medical imaging with applications in radiation therapy, surgery and chemotherapy, and, in particular, to a method and apparatus for real-time tracking of tumor location for improved radiation therapy.
External beam radiation therapy treats tumor tissues by directing a beam of high-energy radiation into the patient and through the tumor, exploiting increased sensitivity of rapidly dividing tumor cells to radiation damage. Radiation, and radiotherapy, as used herein, will generally be understood to include high-energy electromagnetic radiation, as well as particles such as protons.
The success of external beam radiation therapy is strongly linked to the delivered dose, the latter which is constrained by the need to protect tissue and organs near the tumor. Damage to healthy tissue can be reduced by directing the radiation beam along a number of different axes that all intersect at the tumor while reducing dose to the tissue outside of the intersection. In addition, precise collimation of the radiation beam, for example by computer-controlled shutters, can provide sharp dose gradients at the tumor boundary.
Sophisticated external beam radiation therapies rely on a computerized treatment plan that may be prepared, for example, from images obtained on a CT or MRI machine prior to the radiation treatment. In formulating the plan, the tumor and surrounding healthy tissue are “segmented” into defined volumes and each of the volumes is assigned to a minimum dose (for tumors) or maximum dose (for healthy tissue). The process of segmenting tumor volume can be time-consuming, requiring a healthcare professional to draw boundary lines on multiple image “slices” that together describe a volume of tissue to be treated. Semiautomatic procedures for segmentation are available in which the healthcare professional identifies a “seed” in each volume and the volume is automatically contoured by identifying tissue surrounding the seed that is similar to the seed tissue.
During the radiation treatment, ability to precisely control the dose to the tumor is significantly limited by uncertainties in the position and size of the tumor. “Fractionation”, which breaks the radiation therapy into multiple sessions on different days, each delivering a fraction of the total desired dose, may be used to increase the susceptibility of the tumor cells and allow healthy cells to repair between fractions, but can reduce the accuracy of dose placement because of tumor position changes between fractionation sessions. Further, the tumor size may change over time (regression) as a result of the treatment itself.
Even during a particular treatment session, uncertainty in the tumor position can result from periodic organ movements such as those of the heart and lungs or aperiodic motion such as bladder filling.
The problems of tumor motion can be addressed to some extent by monitoring periodic motion (for example with an ECG or respiration sensor) and providing the radiation plan developed for a contour which envelops the tumor motion in all the 10 different phases. Using this approach, a typical radiation plan for lung cancer may include approximately 100 images for each plan. The process of segmenting the tumor in each of these images is extremely time-consuming and prone to error.
A new generation of radiation therapy equipment such as the Renaissance system of ViewRay, Inc. of Cleveland, Ohio provides real-time MR imaging of the patient during the radiation therapy, for example, providing images every quarter second during a 30 seconds to 5 min. radiation therapy session. Ideally, these images could be used to monitor the changing location and position of the tumor during a treatment session. Manual and semiautomatic segmentation is much too slow to exploit this ability. Unfortunately, automatic segmentation techniques such as thresholding, region growing, clustering, and neural networks, have failed to achieve sufficiently accurate or rapid segmentation for real-time tumor segmentation.